1. Field of the Invention
The present invention relates to a process for producing an optical information recording medium transparent sheet (i.e., a transparent sheet for an optical information recording medium), an optical information recording medium transparent sheet produced by this production process, and an optical information recording medium provided with this optical information recording medium transparent sheet.
2. Description of the Related Art
Conventional write-once type optical information recording media, on which information can be written only once by irradiation with laser light, are called CD-Rs and are widely known. Such media have the merit that information can be reproduced by means of CD players already commercially available, and with the recent spread of personal computers, demand for such CD players has been increasing. As media capable of recording a larger capacity of information than that of CD-Rs, write-once type digital versatile discs (DVD-Rs) have also become popular in response to demand for digital high-vision recordings.
As one example of the structure, a write-once type optical information recording medium is known comprising a disc substrate having disposed thereon, in order, a light-reflective layer containing Au or the like, a recording layer containing an organic compound and a cover layer which includes an adhesive layer to allow adhesion to the recording layer. Information can be recorded and reproduced by radiating laser light onto this medium from the side provided with the resin layer. Specifically, the portion of the recording layer that is irradiated absorbs light, and as a result the temperature rises locally at the irradiated portion. The temperature thus raised produces a deformation at the irradiated portion (e.g., formation of pits), whereby information is recorded. The information thus recorded on the media is usually reproduced by irradiating the media with laser light having the same wavelength as that of the laser light used to record the information, and by detecting a difference in the rate of irradiation between the thermally deformed region of the recording layer (recorded portion) and the non-deformed region of the recording layer (unrecorded portion).
Recently, high-vision television and networks such as the Internet have rapidly become more widespread. In addition, HDTV (High Definition Television) broadcasting has begun. Under these circumstances, large-capacity optical information media capable of recording visual information easily and inexpensively are in demand. While DVD-Rs currently play a significant role as large capacity recording media, the demand for media having even greater recording capacity and even higher density continues to escalate, and development of recording media that can cope with this demand is also needed. For this reason, further progress has been made in the development of recording media which have even greater storage capacity, with which high-density recording can be effected by means of short wave light. In particular, the development of write-once type optical information recording media, on which information can be recorded only once, is strongly desired since the frequency with which such media are used is increasing, resulting from the fact that a large capacity of information can be stored for prolonged periods or used as back-up information.
Usually, the recording density of an optical information recording medium may be increased by shortening the wavelength of a laser beam for recording and reproducing, or alternatively by making the beam spot smaller by increasing a numerical aperture (NA) of the object lens. Recently, lasers ranging from red semiconductor lasers having wavelengths of 680, 650 and 635 nm to blue-violet semiconductor lasers (hereinafter referred to as “blue-violet lasers”) having wavelengths of 400 to 500 nm and capable of recording information at a ultra-high density have made advances, and optical information recording media capable of accommodating these lasers have also been developed. In particular, since the blue-violet lasers were launched, an optical recording system that uses a blue-violet laser and a high NA pick-up has been developed and researched, and a rewritable optical information recording medium and an optical recording system having a phase transition recording layer have been developed as a DVR system (“ISOM 2000”, pp 210–211). This system constitutes a certain degree of success in the search for higher recording density in rewritable optical information recording media.
In an optical information recording medium used in an optical recording system utilizing the blue-violet laser and a high NA pick-up, when the recording layer is irradiated with blue-violet laser light for properly focusing the object lens having a high NA, it is preferable to make a cover layer thinner. The cover layer is made, for example, of a transparent thin film, and is adhered to the recording layer with a bonding agent or a adhesive. A thickness of the cover layer including a thickness of the bonding layer or the adhesive layer formed by curing the bonding agent or the adhesive, is normally about 100 μm. This thickness can be optimized depending on a wavelength of the laser irradiated or the NA. However, since, as described above, such an optical information recording medium utilizes a high-NA pick-up, a distance between the pick-up and the cover layer is small, and surface fluctuation of the optical information recording medium forces the pick-up portion into contact with the cover layer. As a result, a problem arises that the cover layer is becomes prone to contact blemishes.
To cope with this problem, a method of forming an blemish-preventive layer or a hard coat layer on the cover layer has been proposed, using spin coating or vacuum depositing, such that the cover layer can be protected from contact blemishes (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-67468 and Japanese Patent No. 3112467). However, this kind of blemish-preventive layer or hard coat layer has been formed and disposed on cover layers on a one-by-one basis, thus posing a problem of low productivity. In cases where such a blemish-preventive layer or hard coat layer has been formed by spin coating, the thickness of the layer has tended to increase toward its outwardmost portion because of centrifugal force, thereby creating a problem insofar that layers produced have been deficient in terms of a thickness precision.